Where seasonal gradients are involved, the pattern of Chla content and leaf morphology can be used to express the ecological changes of species as key indicators of the physiological stage, productivity, and stress of a mangrove forest. In this
investigation, I determined that variability in Chla can occur amongst various mangrove classes found within a degraded forest of the sub-tropics. Specifically, in this study area, the Chla content of all three mangrove species in poor condition showed seasonal dependence, unlike those that were healthy. As previously described by Kovacs et al. (2011), the fringe mangrove of this region of Mexico is typically healthy, whereas basin mangrove is more often found in a poor/dwarf condition. The leaf morphology patterns observed in this study agree with Tomlinson (1986), in that I found bigger leaves in the healthy classes, in particular red and black mangrove, while black dwarf and white poor showed the lowest leaf area and length (Figs. 3 and 4). This is suggestive of a direct relationship between the leaf morphology and the physical state of the trees.
It has been noted that different light and shade requirements in adults of Laguncularia racemosa are indicative of a shade-intolerant response (Smith 1992, McKee 1995), suggesting a pattern in which leaves from the lower canopy and under thicker cover receive less light during the rainy season. As seen with the increase in LAI during the rainy season in healthy stands, this could decrease Chla content in the lower leaves and therefore result in more stress because of the low irradiances as depicted in Fig. 2.2. By contrast, the apparent lack of change in the Chla content in healthy leaves from the upper canopy suggests that the aforementioned shade-intolerant pattern from the lower canopy is present in this type of healthy forest. However, the higher leaf area and leaf length during the rainy season may indicate that at the top of the canopy, the Chla content has no apparent dependence on the morphology of the leaves.
Regarding the white mangrove in poor condition, the shade-intolerant pattern (Smith 1992, McKee 1995) was not observed, as there was no change in LAI. Moreover, the higher Chla content and leaf length during the rainy season indicate that this poor- condition forest is distinctly seasonal in its development. Tomlinson (1986) indicated that for this species, branching occurs during the rainy season, with an extended period of inactivity during the dry season, suggesting that at high irradiances and lack of fresh water, the vegetative survival and competitiveness of Laguncularia racemosa could depend on an efficient display of foliage and the ability to respond to environmental changes and stress.
Regarding the red poor condition, the increases in Chla content and leaf length during the rainy season in both upper and lower canopies suggest that this type of forest greatly depends on fresh water availability and shade as previously reported by
Farnsworth & Ellison (1996). In contrast, the lack of seasonal change within the healthy forest in LAI and leaf morphology suggests moderate sun-shade flexibility. Ellison & Farnsworth (1993) reported that Rhizophora mangle is capable of adapting to different light levels, including gaps within the canopy. It was noted in the field that the majority of healthy R. mangle were found in a continuous stand along the main channel where no other species of mangrove could constrain the availability of light. The lack of observed seasonal change in LAI, Chla, and leaf area in this study would suggest an adaptation of fringe R. mangle to constant tidal flushing.
In this study, a high Chla content was found in the upper leaves of healthy red mangrove during the rainy season. Lugo et al. (1975) indicated that the non-shaded leaves (i.e. upper canopy) of this species might show a net photosynthetic rate twice as high as that of the shaded leaves (lower canopy). Regardless of canopy composition, it has been reported that Rhizophora mangle trees can assume a shade-tolerant (Farnsworth & Ellison1996) and shade-intolerant (Snedaker 1995) pattern. In this study, the red mangrove in poor condition could be indicative of a shade-tolerant pattern, with higher content of Chla in the lower canopy. By contrast, the healthy red mangrove in this study would indicate a more shade-intolerant trend, with higher Chla content in the upper canopy where the availability of light is higher as compared to the shaded leaves in the lower canopy.
The patterns of the black dwarf mangrove suggest a distinctive seasonal pattern similar to the red mangrove in poor condition, with the only difference being no
significant seasonal change in LAI. The low Chla content during the dry season could be the result of increasing soil temperature and decreasing humidity (Sherman et al. 2000), as these trees are typically close or adjacent to drier uplands (i.e. saltpan). Ball & Critchley (1982) reported that shaded leaves of Avicennia germinans can have a higher Chla content during the dry season, suggesting a more intolerant pattern to light
availability (Feller et al. 2007), and thus revealing a high vulnerability to photoinhibition (Cheeseman 1994).
Within healthy black mangrove forest, Gratani (1997) suggested that the major acclimation of leaves in a lower canopy with low irradiance is the development of thinner leaves. In my study, I did not measure leaf thickness. However, the apparent lack of change in leaf morphology may suggest that healthy Avicennia germinans is well adapted to shaded conditions as mentioned by Attiwill & Clough (1980). In the present study, the lack of change in LAI and Chla for the shaded leaves may suggest that this forest does not present significant seasonal changes.
Monitoring the seasonal development of mangrove species and conditions along a mixed environment is important for future research, particularly when dealing with studies that examine remotely sensed data, carbon allocation, or biomass. The observed differences between seasons for some of the species and conditions examined would indicate a clear pattern that this study site is dependent primarily on fresh water availability. Given the large geographic extent and inaccessibility of this type of sub- tropical canopy, remote-sensing image acquisitions are commonly used to monitor and map mangroves. In particular, for degraded systems, remotely sensed imagery is often used to monitor parameters directly related to the LAI and/or chlorophyll content. For example, many estimates of biomass or LAI from remote sensing platforms are dependent on standard vegetation indices (e.g. the normalized difference vegetation index), which are calculated from spectral reflectance directly related to the canopy thickness and leaf Chla content (Jensen 2005). Consequently, knowing the seasonal changes in these parameters would allow remote sensing specialists to identify the optimal time to acquire imagery for accurate biomass or LAI mapping and monitoring. Moreover, collecting these data on an annual basis could be beneficial for monitoring potential impacts on these particular ecosystems resulting from abnormal years of precipitation and/or temperature.
2.5
References
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Chapter 3
3
Assessing the utility of a portable pocket instrument for
estimating seasonal mangrove leaf chlorophyll content.
23.1
Introduction
Both chlorophyll-a (Chla) and chlorophyll-b (Chlb) are considered two of the most important leaf pigments, as they are accountable for the majority of the conversion of light energy into stored chemical energy within plants (Blackburn 2007). Chla is essential in the energy phase of photosynthesis whereas Chlb captures light at a slightly different wavelength (Eichhorn et al. 2005). Moreover, the pigment content variation in total chlorophyll (i.e., Chla+Chlb) between and within species is important for several reasons. First, the amount of solar radiation absorbed by a leaf depends on the foliar concentrations of photosynthetic pigments, and therefore low concentrations of chlorophylls can directly limit photosynthetic activity and hence primary production (Fillela et al. 1995). Second, pigmentation can be directly related to physiological stress because concentrations of chlorophylls tend to decrease under stress and during
senescence (Peñuelas and Fillela 1998). Consequently, quantifying these proportions can provide important information regarding the relationships between plants and their environment.
Traditionally, chemical methodologies for pigment contents are determined using extraction in an organic solvent, which is followed by the spectrophotometric
determination of absorbance by the extracted pigment solution (Wellburn 1994). The actual conversion of the absorbance values to concentration of pigments are then determined using empirical model equations (Hendry and Price 1993, Richardson et al. 2002). Unfortunately, the chemical method of pigment extraction is a destructive process
2
A version of this chapter has been published: Flores-de-Santiago F., Kovacs JM., Flores-Verdugo F. (2013). Assessing the utility of a portable pocket instrument for estimating seasonal mangrove leaf chlorophyll content. Bulletin of Marine Science. 89(2), 621-633.
that can be relatively expensive and, most importantly, time consuming. A recent
alternative to this strategy is the use of non-destructive optical methods for measurement and estimation of leaf pigment contents (Meroni et al. 2009). Optical methods generally yield a chlorophyll index value that expresses, or can potentially be converted to, relative chlorophyll pigment concentration (Goncalves et al. 2008).
The Opti-Sciences CCM-200 Chlorophyll Content Meter is marketed as an instrument that is pocket portable, inexpensive, rapid and easy to use, and provides fast, accurate chlorophyll index readings on the intact leaves of plants, particularly for crops. The CCM-200 avoids the need for grinding or destructive chlorophyll assays (Opti- Sciences 2002), does not require hazardous compounds or specially trained personnel, and the data acquired can be rapidly downloaded for a posteriori computer-based
analysis (Cate and Perkins 2003). Moreover, the CCM-200 could be useful for improving health assessments such as plant stress andleaf senescence (Biber 2007). The CCM-200 operates by differential absorption of light at two wavelengths, one in the near infrared (931 nm), and one through the peak absorbance of chlorophyll (653 nm). Using
calibrated light emitting diodes and receptors, this unit calculates the Chlorophyll Content Index (CCI), which is defined as the ratio of transmission at 931 nm to 653 nm through a leaf sample (Opti-Sciences 2002).
Under conditions of stress (e.g., salinity, drought, high irradiance, and high temperature) plants are often exposed to more radiant energy than is needed for photosynthesis and this places severe limits on plant growth (Ehrenfeld 1990). The mechanisms for disposing of excess energy are limited, manifesting changes within the photosystem as a function of pigments and heat dissipation (Neumann et al. 2008). Mangrove ecosystems are particularly sensitive to periodic, short-term flooding due to coastal tide dynamics (Young et al. 1995) which makes them particularly sensitive to slight modifications in the environment, whether anthropogenic or naturally induced, which can result in considerable stress. For example, Flores-de-Santiago et al. (2012) reported that in a sub-tropical mangrove forest where fresh water availability is extremely seasonal, precipitation patterns affect the leaf Chla content variation resulting in an increase in leaf Chla content during the rainy season in poor/dwarf stands of
Laguncularia racemosa, Rhizophora mangle, and Avicennia germinans. Consequently,
monitoring such stress using a rapid and relatively inexpensive instrument for estimating pigment variation could be extremely useful when trying to assess ecological impacts.
Given the alarming reports of recent mangrove loss (Valiela et al. 2001, Duke et al. 2007, Polidoro et al. 2010) and the importance of these forests for local communities (Walters et al. 2008), organic carbon dynamics (Kristensen et al. 2008), climate change (Gilman et al. 2008), fauna interactions (Nagelkerken et al. 2008), and primary
productivity (Komiyama et al. 2008), a cost-efficient, user friendly, and rapid technique for monitoring pigments could be extremely useful for resource management and scientists studying and/or monitoring mangroves. Consequently, the purpose of this investigation is to determine the feasibility of using CCI, the unit of measure of the CCM-200 portable Chlorophyll Content Meter, as an accurate estimator of the seasonal